In the semiconductor wafer production process and multilayer electronic circuit board construction, numerous individual processes are performed sequentially to construct layers of a three dimensional electronic circuit. The general process depends critically on the alignment of each of the individual processes. To characterize alignment between layers, image primitives called alignment or registration marks are imprinted during each process step. By measuring the relative positions of these registration marks, the registration of layers can be determined. Layers can be mis-registered in x and y position and the two layers can be rotated with respect to each other. The amount of mis-registration that is allowable depends upon the application and the critical dimensions of the electronic circuit that is being constructed. Mis-registration detection is important because of its effects on yield and performance of the finished circuit.
Detection of alignment or registration marks and their accurate characterization may be done manually or automatically. Both processes suffer from corruption of the marks by noise and processing artifacts that cause interference with the basic imprinted marks. Interference creates gray scale patterns that perturb the simple patterns and the background, making it more difficult to measure basic registration information.